The largest ethnic groups in modern times comprise hundreds of millions of individuals (Han Chinese being the largest), while the smallest are limited to a few dozen individuals (numerous indigenous peoples worldwide). Larger ethnic groups may be subdivided into smaller sub-groups known variously as tribes or clans, which over time may become separate ethnic groups themselves due to endogamy and/or physical isolation from the parent group. Conversely, formerly separate ethnicities can merge to form a pan-ethnicity, and may eventually merge into one single ethnicity. Whether through division or amalgamation, the formation of a separate ethnic identity is referred to as ethnogenesis.
Depending on which source of group identity is emphasized to define membership, the following types of ethnic groups can be identified:
In many cases – for instance, the sense of Jewish peoplehood – more than one aspect determines membership.
Ethnic groups derived from the same historical founder population often continue to speak related languages and share a similar gene pool. By way of language shift, acculturation, adoption and religious conversion, it is possible for some individuals or groups to leave one ethnic group and become part of another (except for ethnic groups emphasizing racial purity as a key membership criterion).